This invention relates to high hydrocarbon resistant chemically cross-linked aromatic polyimide membranes and methods for making and using these membranes.
In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications of membrane gas separation have achieved commercial success, including N2 enrichment from air, carbon dioxide removal from natural gas and from enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams. For example, UOP's Separex™ cellulose acetate spiral wound polymeric membrane is currently an international market leader for carbon dioxide removal from natural gas.
Polymers provide a range of properties including low cost, permeability, mechanical stability, and ease of processability that are important for gas separation. Glassy polymers (i.e., polymers at temperatures below their Tg) have stiffer polymer backbones and therefore let smaller molecules such as hydrogen and helium pass through more quickly, while larger molecules such as hydrocarbons pass through more slowly as compared to polymers with less stiff backbones. Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used for natural gas upgrading, including the removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, and in chemical, thermal, and mechanical stability.
The membranes most commonly used in commercial gas and liquid separation applications are asymmetric polymeric membranes and have a thin nonporous selective skin layer that performs the separation. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. According to this solution-diffusion model, the membrane performance in separating a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: the permeability coefficient (abbreviated hereinafter as permeability or PA) and the selectivity (αA/B). The PA is the product of the gas flux and the selective skin layer thickness of the membrane, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases (αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficients because of a high solubility coefficient, a high diffusion coefficient, or because both coefficients are high. In general, the diffusion coefficient decreases while the solubility coefficient increases with an increase in the molecular size of the gas. In high performance polymer membranes, both high permeability and selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given volume of gas, thereby decreasing capital cost of membrane units, and because higher selectivity results in a higher purity product gas.
One of the components to be separated by a membrane must have a sufficiently high permeance at the preferred conditions or extraordinarily large membrane surface areas is required to allow separation of large amounts of material. Permeance, measured in Gas Permeation Units (GPU, 1 GPU=10−6 cm3 (STP)/cm2s (cm Hg)), is the pressure normalized flux and equals to permeability divided by the skin layer thickness of the membrane. Commercially available gas separation polymer membranes, such as CA, polyimide, and polysulfone membranes formed by phase inversion and solvent exchange methods have an asymmetric integrally skinned membrane structure. Such membranes are characterized by a thin, dense, selectively semipermeable surface “skin” and a less dense void-containing (or porous), non-selective support region, with pore sizes ranging from large in the support region to very small proximate to the “skin”. However, fabrication of defect-free high selectivity asymmetric integrally skinned polyimide membranes is difficult. The presence of nanopores or defects in the skin layer reduces the membrane selectivity. The high shrinkage of the polyimide membrane on cloth substrate during membrane casting and drying process results in unsuccessful fabrication of asymmetric integrally skinned polyimide membranes using phase inversion technique.
In order to combine high selectivity and high permeability together with high thermal stability, new high-performance polymers such as polyimides (PIs), poly(trimethylsilylpropyne) (PTMSP), and polytriazole were developed. These new polymeric membrane materials have shown promising properties for separation of gas pairs like CO2/CH4, O2/N2, H2/CH4, and C3H6/C3H8. However, current polymeric membrane materials have reached a limit in their productivity-selectivity trade-off relationship. In addition, gas separation processes based on glassy polymer membranes frequently suffer from plasticization of the stiff polymer matrix by the sorbed penetrating molecules such as CO2 or C3H6. Plasticization of the polymer is exhibited by swelling of the membrane structure and by a significant increase in the permeances of all components in the feed and decrease of selectivity occurring above the plasticization pressure when the feed gas mixture contains condensable gases. Plasticization is particularly an issue for gas fields containing high CO2 concentrations and heavy hydrocarbons and for systems requiring two-stage membrane separation.
US 2005/0268783 A1, US 2009/0182097 A1, and US 2009/0178561 A1 disclosed chemically cross-linked polyimide hollow fiber membranes prepared from two separate steps. Step one is the synthesis of a monoesterified polyimide polymer in a solution by treating a polyimide polymer containing carboxylic acid functional group with a small diol molecule at esterification conditions in the presence of dehydrating conditions. However, significant extra amount of diol was used to prevent the formation of biesterified polyimide polymer. Step two is the solid state transesterification of the monoesterified polyimide membrane at elevated temperature to form a cross-linked polyimide membrane.
Chemical cross-linking of polyimides using diamine small molecules was also disclosed. (J. MEMBR. SCI., 2001, 189, 231-239) However, CO2 permeability decreased significantly after this type of cross-linking. In addition, the thermal stability and hydrolytic stability of the diamine cross-linked polyimide were not improved.
Koros et al. disclosed decarboxylation-induced thermally cross-linked polyimide membrane. (J. MEMBR. SCI., 2011, 382, 212-221) However, decarboxylation reaction among the carboxylic acid groups on the carboxylic acid group-containing polyimide membrane occurred at temperatures higher than the glass transition temperature of the polyimide polymer. Such a high temperature resulted in densification of the substructure of the membrane and decreased membrane permeance.
U.S. Pat. No. 7,485,173 disclosed UV cross-linked mixed matrix membranes via UV radiation. The cross-linked mixed matrix membranes comprise microporous materials dispersed in the continuous UV cross-linked polymer matrix.
U.S. Pat. No. 4,931,182 and U.S. Pat. No. 7,485,173 disclosed physically cross-linked polyimide membranes via UV radiation. The cross-linked membranes showed improved selectivities for gas separations. However, it is hard to control the cross-linking degree of the thin selective layer of the asymmetric gas separation membranes using UV radiation technique, which will result in very low permeances although the selectivities are normally very high.
The present invention discloses a new type of high hydrocarbon resistant chemically cross-linked aromatic polyimide membranes and methods for making and using these membranes.